1. Field of the Invention
The present invention relates to an apparatus for transporting single sheets through a device for exposing or printing single sheets in a processing station. The apparatus includes a drum and pressing means for pressing the single sheet against the drum. The processing station is designed with a processing gap which includes means for limiting the processing gap.
2. Description of the Related Art
DE 196 36 235 A1 discloses an apparatus for recording information on photographic material that can be thermally developed. The apparatus includes a heated drum which deflects each single sheets by approximately 180xc2x0. In a first segment of the deflection, the single sheet is urged against the drum by four pressure rollers. An exposure gap is located between these pressure rollers, with a laser recording on the single sheet through this gap. After the exposure step, the pressing operation is taken over by a continuous belt which wraps around the drum in this stage of the deflection. The continuous belt is provided to maintain the single sheets in close contact with the drum so as to ensure an optimal heat transfer between the drum and the single sheet.
It is the object of the invention to form pressing means for pressing single sheets against a drum, so that single sheets with different formats, in particular with different widths, can be transported past a processing gap with a continuous, jerk-free motion, while simultaneously preventing jamming or canting of the sheets.
The object is solved by an apparatus for transporting single sheets through a device for exposing or printing single sheets in a processing station. The apparatus includes a belt unit for pressing the single sheets against the drum. This belt unit incorporates several parallel extending continuous belts which are mutually decoupled in their movement and which can be driven exclusively directly or indirectly through coupling with the drum surface. Accordingly, different formats can be exposed using, for example, a laser exposure device by decoupling the individual continuous belts of the belt unit. It does not matter if the single sheet is pressed against the drum by all belts of the belt unit or if some of the outer belts make direct contact with the drum. Since each belt can individually adapt to the thickness of the single sheet, the single sheet is also prevented from moving, rotating or slipping.
In an advantageous embodiment, respective belt units are provided in the transport direction of the single sheets both in front of and behind the processing gap, so that each single sheet is pressed against the drum over the entire wrap angle by independent continuous beltsxe2x80x94with the exception of the width of the exposure gap. All continuous belts are decoupled from each other and only driven by the drum, so that the same forces act on the single sheets in front of and behind the exposure gap.
To keep the surface pressure on the single sheets which may consist, for example, of light sensitive photographic paper, as low as possible, the individual continuous belts of the belt units are formed as flat belts.
Each continuous belt is tensioned between two deflection rollers so as to contact the drum surface between these deflection rollers. The two deflection rollers are mounted so as to form an approximately L-shaped tapered gap between the flat belts and the drum surface. A third deflection roller for each of the continuous belts is positioned so as to turn around the portion of the continuous belts which is not in contact with the drum. The position of this deflection roller is not fixed; instead, the deflection roller can move in a direction where the force acting on the deflection roller can tension the belt. In this way, each continuous belt presses with an approximately identical pressure against the drum surface, regardless if a thick or a thin single sheet or no sheet at all is placed between the drum surface and the continuous belts.
Advantageously, the stationarily deflection rollers for the individual continuous belts can be arranged on a common.
In addition, a belt unit for each continuous belts may comprise two stationarily installed deflection rollers and a deflection roller with a spatially adjustable rotation axis. Also, a force is applied to the deflection roller with the spatially adjustable rotation axis in a direction that tensions the continuous belt. The stationarily installed deflection rollers form a respective gap with the drum when the continuous belt is applied, with the gap being wider than the thickest single sheet to be processed. In addition, the continuous belts between the stationarily installed deflection rollers wrap around the drum with an angle of greater than 40 degrees.
Advantageously, the diameter of the drum can be selected to be no less than 150 mm. For smaller diameters, for example, photographic paper could exert a force on the belt units due to its internal stress, which could prevent the paper from reliably contacting the drum.